1. Field of the Invention
This invention relates generally to the field of digital communications networks utilizing high speed packet switching. More particularly, this invention relates to a circuit arrangement for high speed packet switching suitable for minimizing delay in a network which switches voice packets.
2. BACKGROUND
A typical packet switching data communication network is made up of a number of nodes which are interconnected as shown in FIG. 1. The simple network of FIG. 1 includes nodes 10, 12, 14, 16, 18, and 20. Each node includes a number of input ports such as 22, 24, 26, 28, 30 32, 24, 26, and 38. Each node also includes a number of output ports 40, 42, 44, 46, 48, 50 and 52. The nodes are c onnected together by a plurality of links which may be wire transmission lines such as 60, 62, 64, 66, 68, 70, 72, 74 and 76. Such a network is suitable for bidirectional communication of data or voice information as is known in the art. In FIG. 1, only unidirectional communication paths are shown for clarity, but it is understood that for each input port there is a corresponding output port and vice versa.
Each node of the network is connected to one or more users of the network. Thus, packets flowing from node to node in the network may be destined for a variety of users, many of whom may be connected to a single node.
By way of example, if a packet enters input port 22 of node 10 destined for node 20, a number of possible paths may be taken to that node to form a virtual connection between node 10 and node 20. For example, the packet may flow from node 10 to node 12 and then on to its final destination node 20. It may also flow from node 10 to node 16 to node 18 to node 20. One skilled in the art will readily recognize that a number of possible paths may be utilized to complete the virtual circuit from node 10 to node 20. This is advantageous to provide network redundancy and allow for dynamic load allocation. Various techniques are known in the art for determining which route should be taken by the packet. Typically, a routing table is established by analyzing the statistical loading of the network. In the event of a failure in the network, the routing table may be modified so that different paths may be taken to avoid the failure. For example, if a packet destined for node 20 from node 10 normally is routed through node 12, a failure in trunk 62 can be dealt with by changing the routing table to send the packets to node 18 and then node 20. The routing table may also be modified continuously in order to accomplish dynamic load balancing. Of course, other routing techniques such as flood routing may be used.
In a normal packet switching network each node individually looks at incoming packets and analyzes them to determine which outbound port the packet should be transferred to. This can be a time consuming process using conventional technology. When the packets contain digitized voice, any significant delay passing through the node can result in unacceptable speech quality at the packet's final destination. It is therefore critical, especially in a system which handles voice packets, to reduce the delay encountered at each node to an acceptable level. This delay problem is one of the major problems encountered in packetizing voice.
In conventional packet switch systems, incoming packets may be stacked up in a large buffer for analysis by computer prior to being passed on to the next node. This technique may prove unacceptable for voice packets and even for data packets in a system which utilizes extreme high speed transmission of data over a media such as fiber optics. In such situations, it is advantageous to provide packet switching on an almost instantaneous basis as the packet is passing through the node. Otherwise, network congestion will occur unless the packets are processed at extremely high speed within each node. With transmission rates in the range of several gigabits per second, simple hardware which does not require detailed computer analysis or other slow techniques for routing packets is clearly desirable. By utilizing the techniques and hardware of the present invention, a cost effective solution to these and other problems may be realized.